posts not triggering pingbacks

When I link to another blog it does not trigger a pingback on the other site. I have had this problem with a DPChallenge post and now a post linking to several other blogs. Some of the links have generated a pingback and others have not. How can I fix this?

Thank for your response. One of the posts that didn't register a pingback was the DPChallenge. They would definitely have pingbacks on. I don't think that is the issue. I think it's something from my end but I'm not sure what?